Multi-antenna (MIMO: Multiple in Multiple out) wireless transmission technology configures multiple antennas at transmit side and receive side and makes use of space resources in the wireless transmission so as to obtain spatial multiplexing gain and space diversity gain. Information theory research shows that MIMO system capacity linearly increases with the minimum value of the number of transmit antennas and the number of receive-antennas. A schematic drawing of MIMO system is illustrated in FIG. 1. As shown in FIG. 1, multiple antennas at the transmit side and receive side constitute multi-antenna wireless channels including space-domain information. In addition, OFDM (Orthogonal Frequency Division Multiplexing) technology has strong anti-fading capability and high frequency efficiency and is suitable for high-speed data transmissions in a multi-path scenario and fading scenario. MIMO-OFDM technology that combines MIMO and OFDM technologies has become to a core technique of new generation mobile communication.
For example, 3GPP (the 3rd Generation Partnership Project) organization is an international organization in mobile communication fields and plays an important role in standardization work of 3G cellular communication technologies. 3GPP organization started to design EUTRA (Evolved Universal Mobile Telecommunications System Terrestrial Radio Access) and EUTRAN (Evolved Universal Mobile Telecommunications System Terrestrial Radio Access Network) which is also called LTE (Long Term Evolution) project from the latter half in 2004. Downlinks of LTE system adopt the MIMO-OFDM technology. From 3GPP organization Meeting in April 2008 in Shenzhen, China, standardization work of 4G cellular communication system (now called LTE-A system) began to be discussed. During the meeting, a concept “multi-antenna-multi-cell cooperation” has been widely considered and supported and its basic principle is to provide communication services to one user or more users simultaneously by using a plurality of base stations so as to improve data transmission rate of cell edge users.
Generally speaking, in a downlink multi-antenna-multi-cell cooperative system, the user equipment needs to feed back channel state information (CSI) to the base station and then the base station is able to conduct operations such as corresponding wireless resource management. There are three kinds of channel state information feedback methods in the conventional technical Literatures.
Perfect channel state information feedback: the user equipment quantizes all elements in a receive-side channel matrix and feedbacks the elements to the base station one by one; or the user equipment obtains an instantaneous covariance matrix of a channel matrix of the receive-transmit-side, quantizes all elements in the covariance matrix and feedbacks the elements to the base station one by one. Thus the base station can reconstruct relatively accurate channels based on the quantized channels fed back from the user equipment (see non-patent Literature 1: 3GPP R1-093720, “CoMP email summary”, Qualcomm). FIG. 2 is an implementation schematic diagram illustrating this method.
Statistics based channel state information feedback: the user equipment conducts statistic processing for the channel matrix of the receive-transmit side, for example, calculating its covariance matrix, then quantizes the statistic information and feedbacks to the base station. The base station obtains statistics channel state information according to the feedback from the user equipment (see non-patent Literature 1: 3GPP R1-093720, “CoMP email summary”, Qualcomm). FIG. 3 is an implementation schematic diagram illustrating this method.
Code space search based channel state information feedback: the user equipment and the base station predefine a finite set of the channel state information (i.e., a codebook space. A codebook space in common use includes a channel rank and/or precoding matrix and/or channel quality indicator, etc.). When detecting the channel matrix of the receive-transmit side, the user equipment searches for the best channel state information element matched to the current channel matrix in the codebook space and feedbacks an index of the element to the base station. The base station queries the predefined codebook space according to the index and obtains rough channel state information (see non-patent Literature 2: 3GPP R1-083546, “Per-cell precoding methods for downlink joint processing CoMP”, ETRI). FIG. 4 is an implementation schematic diagram illustrating this method.
Among the above three methods, the perfect channel state information feedback method has the best feedback effect, but it has the largest feedback overhead and is difficult to be implemented in a real system. Especially in the multi-antenna-multi-cell cooperative system, its feedback overhead fold increases with increment of the number of base stations and thus leads to high implementation difficulty. The code space search based channel state information feedback method has the least feedback overhead but its effect is bad. The reason is that this method could not depict the channel state accurately so that the transmit-side can not fully utilize channel characteristics for pertinent transmission. However, this method is simple in implementation and the feedback can be achieved with several bits, thus it is widely applied in the real system. In comparison, the statistics-based channel state information feedback method achieves a good trade-off between above two methods. This method can use less feedback amount to accurately depict the channel state when the channel state has obvious statistic information so that desire effects can be achieved.
In the downlink multi-antenna-multi-cell cooperative system, the channel state information feedback method is often combined with a detailed cooperative scheme. In some technical Literatures, there are five cooperative schemes for the downlink multi-antenna-multi-cell cooperation.
(1) Virtual Mimo Technology Based Scheme
multi-cells with multiple antennas are considered as a single-cell MIMO system with more virtual antennas so as to achieve large space multiplexing and space diversity gain. Moreover, repeatedly utilizing single-cell MIMO system mechanism is helpful to reduce implementation complexity of the multi-antenna-multi-cell system. This scheme can adopt any one of above-mentioned three kinds of channel state information feedback methods to feed back the channel state information (see non-patent Literature 3: 3GPP, R1-082501, “Collaborative MIMO for LTE-A downlink”, Alcatel-Lucent). FIG. 5 is an implementation schematic diagram illustrating this scheme.
(2) Single-Cell Independent Operation Based Scheme
A single-cell (base station) configured with a plurality of antennas independently provides services to the user equipment and the user equipment adds data from a plurality of single-cells together and thus obtains high space multiplexing and space diversity gain. This scheme is simple in implementations and its signaling overhead is less. This scheme can adopt any one of above-mentioned three kinds of channel state information feedback methods to feed back the channel state information (see non-patent Literature 4: 3GPP, R1-082497, “Network MIMO Precoding”, Texas Instruments). FIG. 6 is an implementation schematic diagram illustrating this scheme.
(3) A Scheme of Simply Combining Channels of Multi-Cells
From view of the user equipment, channel matrixes from cooperative cells to the user equipment could be added and combined directly so as to constitute a virtual channel. Then the single-cell MIMO technology could be applied mechanically. This scheme can adopt any one of above-mentioned three kinds of channel state information feedback methods to feed back the channel state information (see non-patent Literature 2: 3GPP R1-083546, “Per-cell precoding methods for downlink joint processing CoMP”, ETRI). FIG. 7 is an implementation schematic diagram illustrating this scheme.
(4) A Scheme of Simply Combining Channels of Multi-Cells in Combination of Antenna Selection
On the basis of scheme (3), this scheme firstly performs antenna selection for the cooperative cells and directly adds and combines channel matrixes from the cooperative cells subject to the antenna selection to the user equipment so as to constitute a virtual channel. Then single-cell MIMO technologies could be applied mechanically. Since antenna selection information is included in this scheme, it can partially reduce the feedback overhead of the perfect channel state information, i.e., the user equipment only feedbacks the perfect channel state information of selected antennas. Moreover, this scheme could be combined with above statistics based channel state information feedback scheme or codebook space search based channel state information feedback scheme (see non-patent Literature 5: 3GPP, R1-092102, “MBSFN Precoding with Antenna Selection for DL CoMP”, SHARP). FIG. 8 is an implementation schematic diagram illustrating this scheme.
(5) A Scheme of Reconstructing Multi-Cell Channels in Combination with Transmit-Port Selection
The user equipment and/or the cell semi-statically or dynamically select ports for cell cooperative transmission so as to constitute a virtual channel. Then single-cell MIMO technologies could be applied mechanically. Since antenna selection information is included in this scheme, it can partially reduce the feedback overhead of the perfect channel state information, i.e., the user equipment only feedbacks the perfect channel state information of selected antennas. Moreover, this scheme could be combined with above statistics based channel state information feedback scheme or codebook space search based channel state information feedback scheme (see Chinese patent application No.200910162519.9 (Agent docket number IA094046A), “a downlink multi-antenna-multi-cell cooperative method, base station and user equipment”, SHARP). FIG. 9 is an implementation schematic diagram illustrating this scheme.
It is to be noted that the transmission port in the scheme (5) is a wide concept which covers the concept of antenna in the scheme (4). Therefore, the term “transmission port selection” will be adopted in the following descriptions.
In general, the virtual MIMO technology based scheme (1) considers a global optimum MIMO configuration, which has better performances but with high complexity. Especially when the antenna number of a single cooperative cell is too large, the total antenna number of the virtual MIMO system will fold increase, which causes that the system overhead is too high in each aspect to operate. The single-cell independent operation based scheme (2) reduces the implementation complexity, however, the cell cooperation level decreases and the system performance is bad. The scheme (3) simply combines channels of the cooperative cells, which has the advantage of simple implementation and low feedback overhead and has disadvantage of bad performance. The scheme (4) and scheme (5) utilize the transmission port selection so that better system performance could be achieved, but feeding back indexes of the transmission ports still needs large overhead. Furthermore, how to jointly design the feedback scheme of the multi-antenna-multi-cell cooperative system and the feedback scheme of the single-cell system is a new task.
Therefore, the present invention provides a channel state information feedback method with less overhead for the multi-antenna-multi-cell system with transmission port selection based on the above scheme (4) and scheme (5) and meanwhile considers the joint design of the multi-cell feedback and the single-cell feedback.